Color-plus-clear coating systems formed from the application of a transparent topcoat over a colored basecoat have become increasingly popular in the coatings industry, particularly for use in coating automobiles.
Over the past decade, there has been an effort to reduce atmospheric pollution caused by volatile solvents that are emitted during the painting process. It is, however, often difficult to achieve high quality, smooth coating finishes, particularly clear coating finishes, such as are required in the automotive industry, without including organic solvents which contribute greatly to flow and leveling of a coating. In addition to achieving near-flawless appearance, automotive coatings must be durable and chip resistant, yet economical and easy to apply.
The use of powder coatings to eliminate the emission of volatile solvents during the painting process has become increasingly attractive. Powder coatings have become quite popular for use in coatings for automotive components, for example, wheels, axle parts, seat frames and the like. The use of powder coatings for clear coats in color-plus-clear systems, however, is somewhat less prevalent for several reasons. First, powder coatings require a different application technology than conventional liquid coating compositions and thus, require expensive modifications to application lines. Also, most automotive topcoat compositions typically are cured at temperatures below 140° C. By contrast, most powder coating formulations require a much higher curing temperature. Further, many powder coating compositions tend to yellow more readily than conventional liquid coating compositions, and generally result in coatings having a high cured film thickness, often ranging from 60 to 70 microns.
Powder coatings in slurry form for automotive coatings can overcome many of the disadvantages of dry powder coatings. However, powder slurry compositions can be unstable and settle upon storage at temperatures above 20° C. Some aqueous dispersions are known to form powder coatings at ambient temperatures. Although applied as conventional waterborne coating compositions, these dispersions form powder coatings at ambient temperature that require a ramped bake prior to undergoing conventional curing conditions in order to effect a coalesced and continuous film on the substrate surface. Also, many waterborne coating compositions contain a substantial amount of organic solvent to provide flow and coalescence of the applied coating.
The automotive industry would derive a significant economic benefit from an essentially organic solvent-free clear coating composition which meets the stringent automotive appearance and performance requirements, while maintaining ease of application and performance properties. Also, it would be advantageous to provide an organic solvent-free clear coat composition which can be applied by conventional application means over an uncured pigmented base coating composition to form a generally continuous film at ambient temperature which provides a cured film with good appearance and good performance properties.